1. Field of the Invention
This invention relates to a film comprising a poly-p-phenylene-terephthalamide (herein after called PPTA) and the process for producing the same, more particularly to a PPTA film which is transparent and exhibits excellent mechanical characteristics both in the longer direction of the film (hereinafter abbreviated as the MD direction) and the width direction (hereinafter abbreviated as the TD direction) due to its excellent plane orientation characteristics or a PPTA film which is transparent and exhibits excellent mechanical characteristics, being particularly excellent in mechanical characteristics in monoaxial direction and also having practically satisfactory mechanical properties even in the direction perpendicular to said direction, and also to the processes for producing those films.
2. Description of the Related Art
PPTA has a particularly excellent crystallinity and a high melting point and, due to its rigid molecular structure, has advantageous heat resistance and high mechanical strength, and it is a polymeric material which is particularly attracting attention in recent years. It is also reported that the fibers spun from a concentrated solution showing optical anisotropy exhibit high strength and modulus and they have been already industrially practiced. Proposals about application of the polymer for films still involve a number of tasks in practical performance or manufacturing techniques to be solved. Concerning the method for production of PPTA films, while a polymer of high molecular weight is required, no practical example of its film has yet been known. One of the problems possessed by PPTA films is that its useful polymer of high molecular weight is difficultly soluble in any organic solvents and an inorganic strong acid such as sulfuric acid must be used as the solvent. In order to circumvent this problem, for example, Japanese Patent Publication No. 45421/1981 discloses an attempt in which the polymer is made soluble in an organic solvent by copolymerization of a unit in which halogen groups are introduced into the aromatic nucleus of a liniarly para-oriented aromatic polyamide with an aromatic polyamide having no substituent on the aromatic nucleus other than PPTA, and a film is obtained therefrom. Also, U.S. Pat. No. 3,966,686 discloses films of polyamide hydrazide copolymers or aromatic nucleus-substituted polymers. However, these methods have the drawbacks such that the monomers are expensive to make the cost higher and, besides that the advantageous heat resistance and crystalinity possessed by PPTA are impaired.
On the other hand, when a rigid polymer is dissolved in a solvent, it has been clarified theoretically and experimentaly long before that a liquid crystal is constituted under certain temperature conditions at a certain polymerization degree or higher and a certain concentration or higher [P. J. Flory: Proc. Roy. Soc., A234, 73, (1956)].
In recent years performances demanded for films which are required to have mechanical properties in monoaxial direction, for example, base films for magnetic tapes, etc., are becoming increasingly higher. Accordingly if a polymer solution exhibiting optical anisotropy under such a liquid crystal as mentioned above can be extruded through a slit without disturbance of the orientation of the liquid crystal to obtain a film, it can be expected to be a film having high strength and high modulus. Actually, U.S. Pat. No. 3,767,756 and U.S. Pat. No. 3,869,429 disclose a process comprising extruding an aromatic polyamide solution through a short air layer into a coagulation bath to obtain high physical properties However, according to this process only the mechanical strength in the MD direction of the film is strong, but the mechanical strength in the TD direction perpendicular thereto is extremely weak, is easily bursted and further susceptible to fibrilation. Moreover, since the polymer has a domain structure gathered as liquid crystals, the gathered domain structure remains to give a film which is generally opaque.
Accordingly, various processes for producing a film involving no such problem as mentioned above have been variously investigated. For example, Japanese Patent Publication No. 35088/1982 teaches that a film with good isotropy can be obtained by casting a liniarly para-oriented aromatic polyamide solution having optical anisotropy through a ring die by use of the inflation method in the state of a dope simultaneously in biaxial directions, followed by wet coagulation. However, the film obtained had the drawback of low mechanical properties.
In Japanese Patent Publication No. 5407/1984 and Japanese Laid-open Patent Publication No. 132674/1979, it has been proposed to orient biaxially in the extruded direction and the direction perpendicular thereto during extrusion by extruding an optically anisotropic or optically isotropic dope of a liniarly para-oriented aromatic polyamide through a die while giving mechanically a shearing force in the extruded direction and the direction perpendicular thereto in the die. However, this method had the drawback that the structure of the die is complicated to a disadvantage in industrial practice. Further, in J. Appl. Polym. Sci., Vol. 27, No. 8, P. 2965-2985 (1982), it is proposed to obtain a biaxially oriented film by extrusion of an optically anisotropic dope of PPTA through a ring die onto a conical mandrel coated with oil. However, this film is low in isotropic mechanical strength and, when draft is applied, although the mechanical strength in MD direction is high, that in TD direction is markedly low.
Japanese Patent Publication No. 51248/1980 discloses a film of a polyamide hydrazide copolymer and, while stating clearly that "the content of p-phenyleneterephthalamide units is desirably 40 mol % or lower", describes about a preparation example of a film from an optically isotropic dope containing 5% PPTA as Reference preparation example. The PPTA film disclosed in this reference is low in transparency (namely having small light transmittance), and also has the drawback of containing a large number of voids.
Japanese Patent Publication No. 44957/1978 also discloses a film of a polyamide hydrazide polymer. However, the PPTA film disclosed in this reference, since it is obtained by "instantly" coagulating the dope after a dope exhibiting optical anisotropy is casted, the liquid crystal domain inherent in the optically anisotropic dope remains in the film, whereby the film is opaque and lacking toughness.
Japanese Patent Publication No. 17886/1982 discloses a film transparent and isotropic in mechanical properties, which is prepared by heating an optically anisotropic dope of a liniarly para-oriented aromatic polyamide immediately before coagulation until it becomes optically isotropic, followed by coagulation. This method is original as opposed to the general technical thought of attempting to obtain high performance by utilization of optically anisotropic dope of the prior art, and is successful in alleviating the extreme monoaxial orientation of an optically anisotropic dope and at the same time avoiding the phenomenon that the liquid crystal domain structure of the optically anisotropic dope remains after extrusion and coagulated as such to give an opaque film. However, this method also cannot but use an inorganic strong acid such as sulfuric acid or others as the solvent for PPTA, and also depends on heating as the means for making the dope optically isotropic, and therefore during heating to a temperature before the optically anisotropic dope becomes optically isotropic, decomposition of the polymer cannot be avoided, whereby the problem is involved in obtaining a film of high elongation. Also, only by extrusion of the dope after making it optically isotropic by heating, while the flowing orientation during extrusion can be alleviated as compared with the case of an optically anisotropic dope, the dope becomes very highly viscous, and the very extrusion itself becomes difficult. In fact, the PPTA film disclosed in Example 1 of Japanese Patent Publication No. 17886/1982 has small elongation and the film is transparent but it is not satisfactory with respect the degree of transparency (light transmittance) or flatness of the film, partially because drying step is conducted under no tension.
Further, Japanese Patent Publication No. 17886/1982 discloses preparation of a film transparent and isotropic in mechanical properties by a process comprising heating a solution of a liniarly para-oriented aromatic polyamide immediately before coagulation to a temperature at which said dope becomes optically isotropic, followed by coagulation. However, this film is still lacking mechanical properties, particularly the modulus as required by base film for use in magnetic tape.
In order to obtain mechanical properties in monoaxial direction, heat streching in that direction is generally practiced. However, generally speaking, a liniarly para-oriented aromatic polyamide is deficient in thermoplasticity and particularly a liniarly para-oriented polyamide is readily crystalized, indicating an unclear glass transition temperature, and therefore further heat streching after film formation is almost impossible, whereby it is very difficult to improve the physical properties of the film by heat streching. The film obtained according to the process as mensioned above (the process of Japanese Patent Publication No. 17886/1982) can be heat streched with difficulty.
As described above, since an optically anisotropic dope has a domain structure in which the polymers are gathered as liquid crystals, its domain structure remains when the dope is formed into a film and the resultant film becomes opaque. If an optically isotropic dope is used for avoiding this phenomenon, a useful PPTA of high molecular weight becomes very highly viscous to make film fabrication impossible unless at a polymer concentration of about 5 weight % or lower, and the film prepared from an optically isotropic dope of such a concentration is liable to form void during coagulation and is also entirely unsatisfactory with respect to mechanical properties.